Numerous such detectors have been developed of which DE 4203967 A1 (Endress & Hauser) is but an example, but because of the widely varying characteristics of materials to be detected, none has proved entirely satisfactory under all circumstances. Some designs employ forked probes, which can fail to function properly if lumps of the solid to be detected become stuck in the fork. Others, using concentric or single probes, can fail to function reliably if adherent materials or dust or other deposits from the materials build up excessively on the probe assembly. The vibration of the probe can in some cases cause a cavity to form in solid materials around the probe, thus giving a false indication. Careful design is necessary to prevent vibration from the probe being coupled excessively and unpredictably through the walls of the vessel in which the probe is mounted, possibly giving rise to unreliable operation.
In U.S. Pat. No. 4,841,565 (Woodward), assigned to the present applicant, it is disclosed that in an acoustic pulse-echo ranging system, the amplitude of ringing of an acoustic transducer may be sampled a predetermined time after application of a burst of high-frequency electrical energy to the transducer and compared with a threshold value in order to determine whether the transducer is present and operative. In U.S. Pat. No. 4,540,981 (Lapetina), a somewhat similar principle is utilized to determining whether a probe or diaphragm is submerged in liquid. The probe is located within a pipe into which the liquid flows, and is excited from outside of the container being monitored. The ringing o the probe or diaphragm is more heavily damped in the presence of liquid, and this can be detected.
U.S. Pat. No. 5,578,994 of B. Cherek and J. E. Gillis describes a vibrating-fork type level sensor for liquids where the transducer is connected in a feed-back loop of a Butler oscillator operating in a continuous (non-pulsed) mode. A liquid immersion of the sensor shifts or suppresses the dominant frequency mode and reduces its Q-factor causing the oscillator to shut down.
When solids are being sensed, changes in damping applied to a vibratory member may be less marked, depending on the nature of the solid, and thus more difficult to detect.
British Patent Application 2,306,003A, of the present applicant, discloses a vibrating element driven by a transducer, and utilized for sensing the presence of solid materials in contact with the probe, the probe being energized by successive bursts of energy, reduction of the decay time of the vibration envelope of the bursts being indicative of the probe's immersion.